I've owned hermits (properly) for 5 years now. They are social, so always have a few of the same species in your tank. In pet stores you can generally find Purple Pinchers and Ecuadorians. If you order online you can get the more exotic species. PPs are the ones that can get softball sized. E's are generally much more active but supposedly more skittish than the PPs (mine seem equal in that regard).

For a setup you'll need a tank with a lid that covers it completely (they are escape artists), size depends on how many you have and how large they are (and how long you want the tank to last). For substrate a mixture of sand and coconut fiber (eco-earth or bed-a-beast) is recommended, though either can be used by itself too, they just each have possible drawbacks. The substrate should be moist, we say "sand-castle consistency", basically it should easily hold it's form for when they tunnel in it. The substrate should be about three times as tall as your largest crab so they can completely bury themselves where the other crabs can't get them when they molt. You need to monitor temperature and humidity, you want about 80 degrees (F) and 80% humidity. I have a LOT of trouble with humidity, right now I need to mist daily to keep humidity in an acceptable range. For heat I used an infrared heat bulb over the tank.

They need both fresh water and salt water. Both should be de-chlorinated using the same sort of stuff you use for fish tanks. For salt I use Instant Ocean, I think Oceanic is another good brand. Don't by any salt water targeted for hermit crabs, they are generally not salty enough. Bathing used to be recommended but it is now generally believed to be super stressful for them. So instead you should provide water bowls that are deep enough for the crabs to fully submerge and bathe themselves. Of course you may need to add something so your smaller crabs can get out of the bowls. I use tupperware for bowls.

They need extra shells to move into when they grow, you'll find some shells are more popular than others depending on species, get lots of turbos. Like Julee said, painted shells are bad because crabs tend to munch on the paint.

They molt underground periodically. So they shed their exoskeleton and harden up a new one. They are vulnerable at this time so there can sometimes be deaths, sometimes another crab will kill a molter, that's why ample substrate is important. Occasionally a crab will not bury itself to molt, so it should be put into some sort of isolation. Crabs will also go underground if they are stressed (new crab, bad environmental conditions) or just to dig for fun.

Crabs love to climb, so give them lots of wood. When they are mad they chirp so if you hear noise go check it out and make sure no one is getting into a fight. They can be sexed, it's just a little difficult to get them to stretch out of their shell enough to see. They have the potential to live into their 30's but they are pretty sensitive so I don't think it happens often. I lost my two oldest last year when my tank flooded

I think those are the big things. If you have any more questions let me know, I love to talk crab!